EJ: *First, be very careful when defining morality by majority rule. 200 years ago, the majority thought that black should be enslaved, as they were sub-par. That doesn’t make it moral, that means that the victims in question are out-numbered and have little say. *
True, morality isn’t defined by majority opinion. However, the existence or absence of a moral consensus on a particular topic determines whether or not you can assume that most people share your individual moral views. If Debaser wants to assert that progressive taxation is immoral, then since most of the rest of us don’t believe it’s immoral, his assertion isn’t going to carry any weight unless he can actually convince us to agree with him.
Second, the reason most people believe it’s okay to tax the hell out of the rich is that most people aren’t rich.
You seem to have missed the part where I pointed out that plenty of us also think it’s okay for our own taxes to be higher than those of poorer people. If, as you suggest, it’s all just about greed, then how do you explain that?
Also, I could easily retort that the reason people of your viewpoint argue against progressive taxation is that they’re selfish and callous. But I think that that kind of reduction of arguments to interested personal motives—“You only believe A because you’re grasping and greedy!” “Well, you only believe not-A because you’re selfish and callous!”—is pretty tedious and not very illuminating. I prefer to believe that most people are doing their best to reason honestly and fairly according to their lights, and the chief point of debate is to explore how and why honest and fair people can differ in their conclusions.
*Uncle Sam gouges the wealthy, and showers the poor in subsidies and welfare programs - what’s to complain about, if you’re on the receiving end? *
Gee, almost makes you want to be poor so you can get all those great subsidies, doesn’t it? This sort of claim comes across as kind of funny, because it completely ignores the fact that even after we’ve finished “gouging” the wealthy and “showering” the poor, the wealthy are still orders of magnitude better off. In other words, even with “subsidies and welfare programs”, life for the poor is generally full of hardship and struggle, while even with “gouging” taxes, life for the wealthy is still full of privilege and luxury.
In other words, the rich can quite easily afford to be taxed more than the poor. And our general moral consensus is that the moral status of taxation depends not just on the absolute amount of the tax, or on the ratio of tax to income, but also on how easily the taxed person can afford the tax.
This seems to me like a much more reasonable and sensible moral standard than simply requiring that everybody should make the same financial contribution, either as a dollar amount or as a percentage of income. (Added in preview: this is a response to Apos’s question too.) If a certain percentage of income makes the difference between survival and privation to a poor person, while the same percentage makes only a negligible difference in the standard of living of a rich person, then there’s nothing particularly “fair” or “moral” about taxing them the same percentage.
Apos: What bothers me is that redistributionists have absolutely no measure for exactly how much it is “moral” to take, and why.
Well, if the moral justification of progressive taxation is based partly on need and affordability, obviously there is no one figure that’s going to apply across the board. What people need and what they can afford inevitably change with time and circumstances. But personally, I’ve never heard of any progressive-tax supporter (at least, one who wasn’t advocating outright socialism) complaining that the 1950’s top marginal tax rate of 91% was too low, so perhaps you can think of that as a loose upper bound. 
That is, if carpenters didn’t have all those benefits, if their jobs and educations and so on were much harder and more expensive… the “rich” would simply have to pay them more for their work, covering in proportion all the hidden costs of production, transport, education, etc.
That doesn’t seem to make sense: a rich employer isn’t going to pay a future carpenter for the education he’ll need in order to become a carpenter. If we don’t provide benefits to the non-rich from tax revenues, how do we ensure that the non-rich get the basic standard of living that enables them to do the jobs, and consume the products, that contribute to the wealth of the rich?
What you describe as “efficiency” in paying for only what you want, only when you want it, could also be termed a “race to the bottom”. Sure, we can say that we won’t use tax revenues to support universal education, and instead the carpenter and other workers will charge enough for their work to cover the education of their children. But then the workers with few or no children will undercut the price of workers with several children, and the latter then have to cut their price in order to stay competitive. Result: the employers save money, but there’s less money for educating workers’ children. And so on. Will workers in such a “race to the bottom” continue to be adequate producers and consumers, or will the growing poverty of the non-rich adversely impact the rich?